A parallel-kinematic machine of this kind described in U.S. Pat. No. 6,301,525 (corresponds to SE 512 338) includes a universal joint in the form of a gyro suspension that includes a central through-passing hole which is intended to receive an axially displaceable arm which is connected at one end to a positioning head.
With the intention of enhancing the accuracy of positioning head settings in space with regard to the aforesaid type of machine, its universal joint is provided with two angle sensors. A first angle sensor is connected to the first shaft of the universal joint and a second angle sensor is connected to the second shaft of said joint. These sensors detect the angular position of the gyro rings of the joint. The arm carries a length sensor which detects the axial position of the arm in the universal joint.
The angular positions of the gyro rings included in the gyro suspension in relation to a foundation are read-off with the aid of scale markings disposed on a graduated surface, which is either in the form of a curved cylindrical surface or a plain circular surface within each angle sensor, wherein the graduations define a curved scale. The radius of each angle sensor on the curved scale is normally less than 50 mm. Because the angular resolution is inversely proportional to the scale radius, the relatively short radius of this scale results in low resolution with regard to positioning head accuracy.
It is not technically and economically realistic to further enhance the resolution of the sensors by simply increasing the scale radius, since the errors that result when mounting the scale and the errors that result from scale manufacture will increase in proportion to an increase in the radius, making further enhancement of the accuracy unachievable in this way.
Due to the difficulties experienced in fitting the cylindrical surface concentrically with the shaft of the gyro ring, an angular error is also obtained between the cylindrical surface and the shaft pivot.
At these small radii and tolerances vibrations from bearings and guide rails, which have a size in the order of 0.5 μm, are sufficient to introduce errors into the system.